Fire detecting system with spurious signal rejection

ABSTRACT

A fire detecting system having a plurality of fire detectors connected across conductors for applying a voltage thereto. The detectors each include switching means which is closed upon operation of a detector and opened upon reduction of the voltage on the conductors below a predetermined value. Voltage is supplied to the conductors by means which responds to the actuation of one or more detectors to automatically lower the conductor voltage below said predetermined value and then raise it again. After a predetermined number of cycles of operation an alarm is automatically actuated. In this way momentary actuation of a detector will not sound an alarm and thus false alarms are eliminated.

United States Patent 1 1191 Kobayashi et al.

1111' 3,775,761 [451 Nov 27, 1973 FIRE DETECTING SYSTEM WITH [54] 3,621,262 11/1971 .Lecuyer", 340/228 X SPURIOUS SIGNAL REJECTION 39191836 3/1973 3,694,670 9/1972 [7 Inventors: Akihiro b y Fujisawa-shi; 3,503,067 3/1970 Amiragofi' 340/412 Akira Yokota, Tokyo, both of Japan' [73] Assignee: Nittan Company, Limited, Tokyo, Examiner-John Caldwell Japan Assistant Examiner--W1lliam M. Wannisky Att0meyEugene E, Geoffrey, Jr. [22] Filed: Mar. 30, 1972 v [21] Appl. No.: 239,633 [57] ABSTRACT I A fire detecting system having a plurality of fire detec- [30] Foreign Application Priority Data tors connected across conductors for applying a volt- AprxlS, 1971 Japan .Q 46/23446 age thereto The detecwrs each include Switching 1 means which is closed upon operation of a detectorv 52 us. c1. 340/228 R, 340/411, 307/117, and Opened "P reduction of thevolmge on the 7 5 M ductors below a predetermined value. Voltage is sup- [51] Int. Cl. .I G08b 17/06 plied to the by means which responds to [58] Field of Search..... 340/228, 228.2, 412, the actuation of one or more detectors to automati- 340/414, 213, 2131, 307/252 J, 246 cally lower the conductor voltage below said predeter- 2 25 K 5 M, 310, 116, 117 mined value and then raise it again. After a predeter- 1 mined number of cycles of operation an alarm is auto- 5 References Cited matically actuated. In this way momentary actuation UNITED STATES PATENTS of a detector will not sound an alarm and thus false alarms are eliminated. 3,147,464 9/1964 Splelman 340/213.l X 2,903,682 9/ 1959 Van Driel' 340/2282 X 2 Claims, 4 Drawing Figures iv 1* 5 1 /5 /'/27 A l I i 39 CENTRAL UNIT ing a high degree of reliability.

FIR D TE NG TH SPURIOUS $I NA RE E TI This invention relates to a tire detecting system hav- Some of the fire detecting systems whichhave been widely used include fire detectorseach having a controlled switch, such as a silicon controlled rectifier (hereinafter referred to as SCRU, which is closed in response to actuation of the fire sensing; means but is not opened again'unless the voltagethereacro ssis reduced below'a specific value; such detectors are advantageous in that the alarm is continuous eventhough the fire maybe sensedzfor a short time, but they also have an attendant disadvantage in that a; continuous alarm is provided when the detectoris actuated erroneously by amomentarily inducedvoltage, a short pass of (Smoke thrsus i dt t r' SPsiQs QsHSW wind, or any other cause otherthan fire. uch an erroneous alarm often cai ses substantial confusion and,

'therefore, is undesirable."

' alarm onlywhen thepresenceoffire is ascertained.

Should the systetnbe provided withmea'ns for immediately restoring an actuated 'det ector to its original condition, the v detector would be repeatedly actuated and restored until the cause of actuation isrenioved. the above mentioned causes oferroneous actuation are generally momentary and disappear quickly, an erroneous alarm can be avoided fin accordancewith this [in- .vention by energizingithe alarm device after a specific number of repetitions ofactuationa'nd restoration."

[The fire detecting system in this invention is embodied includes a plurality offire detectors connected in parallel betweena pair of conductors and a e entral unit connectedto one end :of saidconduc'tors and eachof the detectors have a normally open switch which is connected between said conductors and closed in response to detectionoffire to short both the conductors. Accordingto' this invention, the central unit includes a dc. voltage source, 'a multivibratorhaving stable and unstat ile'states, nieans for co to number of times the multivibr" is operated and means for producing an outpu I I r number'of operations'is detecte An alarm device is energized by said outputfand acircuitconnection is provided for supplying an operating" voltage to said conductors through a path isconductive during the stable state of said'multivibrator and for driving said multivibrator into the unstable state to malte said path non-conductive when the conductors areshorted through any of the detectors; v V

"Other objects and features of this invention willbe more clearlyunder'stood frorn the following description and accompanyingdrawingslforming part of this application. i

' ln the Drawings:

.FIG. 1 is a schematic circuit diagram of one embodiment of a system in accordance wi th this invention, iwFlG. 2 is a simplified circuit diagram of the voltage stabilizing circuitof FIG. 1, and I "FIGS. 3 and 4 are waveform diagrams which will aid in understanding the operation of the system of FIG. 1.

i Throughout'the drawings like reference numerals are used to denote lilte structural components.

Referring to FIG. 1, the tire detecting system accordingly thi invention includes a central unit 1 having a a o oi .7 detectors 2-11, 2- 2, connected in parallel between put terminals land 4, and a plurality of tire a pair of conductors 3 0and extending from the output terminals Sand The fire detector 2 includes a fire sensor whichsenses a product of fire, such as smoke or temper ure, to produce an output and a normally open switehesnch an SCR 26, is closed by'the output of thefsensor to short the conductors 30 anddt) and is never restored to its non-conductive state unless the voltage qthereacross Y -is reduced below a specific va ue, 7 Thecentral unit comprises a power source section 5,

a multivibrator section 6,an integrating circuit section -7 and an alarm section ,8. I

- The po er source section ,5 includes a dc. voltage source rolsucn abattery and a normally closed pushbptton switch 11 connected in series with the voltage I source and used for restoration of the system when actuated. f i v it e multivibrator section ,6 includes resistors 12, 13, .14 and ]1; 5;capacitors .16 and 17, NPN transistors 18 and 19 and diodes 20and The collector of transistor is'connected through the resistor 12 to the positive terminalof the power source, the emitter is connected to thenegative terminal of the power source and the negative output terminal 4 of the central unit 1, and thef bagseis connected to the cathode electrode .of the diode 2l which has its anode electrode connected through thieresistor 14 to the positiveterminal ofthepower'source. Ihetransistor 19 has its collector connecte to thepositive terminal of the power source throughthe resistor 15, its emitter connected to the anod felectrodeof diode 2 3, and its base connected to 1 Yr; thjode electrode of the diode 20 which has its anodefelec'trode connected through the resistor 13 to the positive terminal of the power source. The capaciis'ieonnectedbetween the junction A of the resistor l 2andt' he transistor 18'and the junction B of the resistor :"13 and the diode 2 0,and the capacitor 17 is between the junction C of the resistor 14 and the diode 21 and the junction D of the resistor 15 transistor 19- The base electrode of the transistor also connected through a resistor 22 to the negative terminal of the power source and the base electrode of thetransisltor 19 is also connected to the cathode electrode of Zener diode 24 which has its ode connected to the negative terminal.

e an deelectrodeofthe diode 23 is connected to K ve outputterrninal 3 of the central unit. In this ator 'section 6,1the resistors 13 and 15, the I or l 9 and the diodes 23 and 24 enclosed by brokenhnesi) constitute a known stabilized voltage source for the fire detectors cooperation with the power source section 5 and is shown more clearly in FIG. 2.

The integrating circuit section 7 includes Zener diodes 27 and 31; capacitors 28 and 32 and a resistor 29. The Zen er diode 27 has its cathode connected to the positive terminal of the power source and its anode to thejunction l). One end of each capacitor 28 and 32 connected to the positive terminal while the other end ofthe capacitor 28 is connected through the resistor29 to the junction and the other end of the capacnets: is connected to the cathode electrode of the Zener diode '31 which has its anode electrode connected to the other end of the capacitor 28.

The alarm section 8 includes a PNP transistor 33 having a base electrode connected to the cathode electrode of the Zener diode 31, an emitter electrode connected to the positive terminal of the power source and a collector electrode connected through a series connection of resistors 35 and 36 to the negative terminal of the source. An NPN transistor 37 has a base electrode connected to the junction between the resistors 35. and 36, a collector electrode connected through a parallel connection of a relay unit 41' and an audible alarm 42 to the positive terminal of the source, and an emitter electrode connected directly to the negative terminal thereof. This section further includes a resistor 34 connected between the emitter and base electrodes of the transistor 33 and a resistor 38 connected voltage is supplied from the voltage source 10 through the voltage stabilizing circuit 9 included in the multivibrator section 6 to the respective fire detectors 2-1, 2-2, connected in parallel between the conductors 30 and 40. The voltage stabilizing circuit 9 which is schematically shown in FIG. 2 is of a type well-known in the art and therefore further description is not deemed necessary. On the other hand, as the emitter electrode of the transistor 18 is directly connected to the negative terminal of the source, a current flows through the resistor 14 and the diode 21 into the base electrode of the transistor 18 and keeps the transistor 18 in the ON condition. Accordingly, currents flow through the resistors 12 and 14 respectively. However, as the emitter of the transistor 19 is connected to the negative terminal of the source through the very high impedance of the fire detectors, little current flows through the resistors 13 and 15. Therefore, the capacitor 16 is charged up to the Zener voltage of the Zener diode 24, making the junction B positive, and the capacitor 16 is charged up to the source voltage, making the junction D positive.

When a fire is sensed by the sensor 25 of one of the tire detectors and the corresponding SCR 26 is driven into conduction, the emitter electrode of the transistor 19 is shorted through the SCR 26 to the negative terminal of the source 10, Thus, the transistor 29 is driven into the ON state and the positive terminal of the charged capacitor 17 is connected to the negative terminal of the source 10. Consequently, the potential at the base electrode of the transistor 18 which is connected to the negative terminal of the capacitor 17 through the diode 21 is momentarily lowered to the potential of the emitter. Therefore, the transistor 18 is driven into the OFF state and exhibits a high impedance.

In this condition, the capacitors l6 and 17 are discharged through the ON transistor 19 and, thereafter, charged again in the opposite direction, that is, the

junctions A and C become positive. When thebase potential of the transistor 18 is raised up to a specific value during this charging process, the transistor 18 is driven again into the ON state and exhibits a low impedance. Therefore, the positive terminal A of the charged capacitor 16 is lowered to the negative terminal potential and, consequently, the base potential of the transistor 19 is also lowered momentarily to the potential of the emitter. Thus, the transistor 19 is driven again into the OFF state and exhibits a high impedance. At this time, the voltage across the Zener diode 24 is also reduced to about zero volts and, therefore, the output voltage applied between the conductors 30 and 40 is similarly reduced to restore the actuated SCR 26 of the fire detector concerned to its original open state. Thus, the system is restored to its original condition.

If the cause of the actuation of the detector remains after the completion of the above mentioned cycle of operation of the system, the same operation is repeated until the cause of actuation is removed. Every cycle of such operation, detected by the voltage across the resistor 15 which is produced in response to conduction of the transistor 19 is accumulated successively in the capacitor 28 with a delay determined by the resistor 29 and the capacitor 28 and, every time the voltage across the capacitor 28 exceeds the Zener voltage of the Zener diode 31, it is transferred to and accumulated in the capacitor 32. The Zener diode 27 connected across the resistor 15 serves the function of a voltage limiter to pass a voltage higher than its Zener voltage but to block undesirable low voltages.

FIG. 3 represents the voltage waveform appearing across the resistor 15. In the drawing, voltages V V and V, are the voltages for restoring the detector,

namely, the voltage at a ready condition and the voltage at an actuated condition. Time points t t and correspond to actuation of the detector, conduction of the transistor 19 and restoration of the detector, respectively. During the repeated actuation of the detector, the voltage across the capacitor 32 is raised successively as shown in FIG. 4 and tends to let a significant amount of current flow through the resistor 34. When a sufficient voltage drop is obtained across the resistor 34, the transistor 33 is driven into the ON state and produces an output at the junction of the resistors 35 and 36. This output is amplified by the transistor 37 and supplied to the relay unit 41 and the audible alarm 42 to energize them. The series connection of the resistor 38 and the diode 39 serves the function of facilitating the voltage drop across the resistor 34 and maintaining the operation of the alarm section 8.

In order to stop the alarm and restore the system to the original condition, the pushbutton switch 11 in the power source section 5 is pushed to interrupt the source voltage supply for a moment.

When the cause of actuation of the detector is a momentarily induced voltage or sudden gust of wind which is not a product of fire, such cause does not continue very long and generaly disappears quickly. Therefore, the accumulation of an electric charge in the capacitor 32 does not become enough to drive the alarm section 8, and the alarm is not operated.

As described in the above, the system of this invention can distinguish the cause of actuation of the detector and give an alarm only when an actual fire occurs.

What is claimed is:

l. A fire detecting system comprising a plurality of fire detectors connected in parallel between a pair of conductors, a central unit connected to one end of said conductors, each of said detectors having a normally open switch connected between said conductors, said normally open switch being closed in response to actuation of said detectors and restored only by the reduc- 2. A fire detecting system according to claim 1 wherein said first means includes a multivibrator having stable and unstable states and initiating operation in response to closure of said switch, and a circuit connection for supplying said operating voltage to said detectors through a path in said multivibrator which is conductive during said stable state but nonconductive during said unstable state. 

1. A fire detecting system comprising a plurality of fire detectors connected in parallel between a pair of conductors, a central unit connected to one end of said conductors, each of said detectors having a normally open switch connected between said conductors, said normally open switch being closed in response to actuation of said detectors and restored only by the reduction of a voltage between said conductors below a specific restoration voltage, said central unit including a voltage source for supplying an operating voltage to said detectors, first means responding to closure of said normally open switch to produce an output voltage and reducing said operating voltage below said restoration voltage, second means for accumulating said output voltage of said first means, third means for producing an alarm in response to a predetermined magnitude of said accumulated voltage of said second means.
 2. A fire detecting system acCording to claim 1 wherein said first means includes a multivibrator having stable and unstable states and initiating operation in response to closure of said switch, and a circuit connection for supplying said operating voltage to said detectors through a path in said multivibrator which is conductive during said stable state but nonconductive during said unstable state. 